Many of us, myself included, have used Asia Miles at the airport. The proper term in AM/MPC parlance is an "Instant Upgrade". Translation for burning Asia Miles at the airport with the ground staff. The check-in agents can process, they give you a physical receipt to sign, and you get a copy.

For waitlist, once you hit departure day the MPC/AM waitlist clears out. Control is handed off to the airport; this means a new waitlist is formed for "Instant Upgrades". It is similar to the AM/MPC waitlist in that DM has priority over GO, over SL, over GR, over AM. The annoying thing is the "Instant Upgrade" waitlist does not see the MPC/AM waitlist, and hence even if you were the top of your elite group on the AM/MPC waitlist, that position won't transfer to the airport waitlist. Instead, the Instant Upgrade is a brand-new waitlist to itself. Which means, you have an incentive to get to the airport early if you're planning on doing Instant Upgrade.

It sounds like you're no status just Asia Miles on CX, right? The example would be this. Say you're on the waitlist, and Asia Miles says you're "first" a month out, ahead of all other non-elite Asia Miles members on the waitlist (DM, GO, SL and GR will still outrank you no matter when they add themselves). Now let's say you don't clear, and you go to the airport super late. Maybe 4 seats were available in J, and there are 1 DM, 2 GOs, and 1 other Asia Miles member on the Instant Upgrade waitlist, all ahead of you. Unfortunately, since that other Asia Miles member got to the airport first, he will get it - not you. Your position ahead of that other Asia Miles member on the old waitlist is irrelevant.

Finally, most outports I believe will allow you to upfare relatively easily. I think it might be a strategy to consider to hold off on buying W/R, go to the airport, and if biz seats are available offer to upfare to W/R on the spot, and then burn AM. However this maneuver could be just too complicated and also bears a bit of risk.